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Advertised battery capacity vs useable capacity

Discussion in 'Electric Vehicles' started by widodh, Jan 24, 2016.

  1. widodh

    widodh Model S R231 EU

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    We all know that you can not use 100% of the advertised battery capacity of any EV.

    A 24kWh Leaf gives you about 22kWh of usable capacity, but it is rumoured that a 85kWh Model S battery is actually 81kWh and roughly 74kWh is usable.

    It might be to soon, but should we (EU and US) should come up with legislation where EV manufacturers have to provide the available capacity for driving?

    I don't want to make this discussion Tesla specific, but I'm curious what others think.

    Should we as consumers be told how many kWh we can use for driving?
     
  2. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    I don't see the need for regulation:

    1. It's impossible to measure the exact SOC without degrading the battery.

    2. Any manufacturer that cheats too much is going to get hammered by reviews and complaints.

    3. There will always be some individual car variance of some small amount.

    4. What happens if the car shuts down with 2 "usable" kWh left? I don't think you can say accurately, in every circumstance, what the absolute usable number is because there are other variables.

    5. To take an example from another industry, drive manufacturers have used 1000 as a MB rather than 1024. Because every manufacturer does this and everyone knows they cheat, most people understand what they are getting (and most of the ones that don't, never come close to filling the drive anyway).
     
  3. apacheguy

    apacheguy Sig 255, VIN 320

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    This should be a consideration for the EPA when determining rated range. BTW, I recall ~81 kWh as the number being floated around on this forum after the EPA conducted their tests. But that was useable capacity. Obviously, the EPA cannot measure total capacity.
     
  4. Saghost

    Saghost Active Member

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    I don't see any real reason for it, because the number of kWh doesn't actually matter to consumers AFAICT. What matters is either how far you can go on the battery or how much it costs to go a given distance. Both of those are already covered by EPA testing, and manufacturers are required by law to use EPA numbers for those.

    The number of kWh is of academic interest as an engineer - but the 265 mile range and the 38 kWh per 100 miles from the wall are the two things that matter in choosing a car.
    Walter
     
  5. widodh

    widodh Model S R231 EU

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    But if a company sells a vehicle with say a 50kWh, it could be that just 40kWh is usable and you are fine with it?

    On the specs page I would want to see:

    Battery: 50kWh (40kWh usable for driving)
     
  6. green1

    green1 Active Member

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    Maybe they shouldn't list the capacity at all, and only say that it can drive x distance on a charge and that you can expect an efficiency of y watthours/distance
    After all, very few ICE vehicles talk about how big their gas tank is. They talk about efficiency, and distance between fill-ups.
     
  7. techmaven

    techmaven Active Member

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    I agree that there should be some standards for this, but it easily becomes messy. Likely Tesla is using the amount given by Panasonic's datasheet. Depending on how you discharge the cell, the amount of energy you can extract changes. Temperature also changes the amount. So you basically have to come down to some sort of standard environment. But each vehicle subjects the cells to different treatment, so even then it won't be accurate. The best we have is the EPA 5 cycle range... which takes all this into account.
     
  8. J1mbo

    J1mbo Member

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    Why? What benefit is this information? Some people get more range out of 40kWh than others can from 50kWh. So, not range. What then....?
     
  9. widodh

    widodh Model S R231 EU

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    So why do we still list horsepower or KiloWatts for power? Isn't the 0-100 (0-60) times and the topspeed enough?

    I do think that the battery size matters. I want to know what I buy.
     
  10. qwk

    qwk Model S P2681

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    Anything but usable energy will be a marketing gimmick, kind of like Tesla's HP claims. What good is a fantasy number when it cannot be accomplished?
     
  11. apacheguy

    apacheguy Sig 255, VIN 320

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    Kwh is a precise unit. Rated range is not. The kWh battery capacity provides a useful comparison between vehicles.

    I would even argue that Tesla should display kWh on the dash when "Energy" is selected as the unit. Percentage is not a unit of energy.
     
  12. green1

    green1 Active Member

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    There are valid reasons, but this really isn't one of them.
    Do you want the car with 60kwh battery, or the one with 55kwh?
    I don't know, I want the one that drives further, and I don't plan to do my own analysis of drag coefficients, inverter losses, rolling resistance, etc.
     
  13. McRat

    McRat Active Member

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    Given two battery assys with identical cells.

    One is 75kWh, one is 60kWh. Both are configured for 50kWh normal operating window.

    The 75kWh can produce 25% more discharge amps, and can charge 25% faster than the 60kWh. It's lifespan will be 25% longer normally, especially if the factory can open the window with software to "refurbish" the battery.

    So if you had 2 cars advertised at 50kWh, and one was 250HP and the other was 225HP, you'd assume Car 1 had a higher stressed battery that would not last as long, which would not be true, it would be the opposite. Fast charging would have smaller effect on Car 1 as well.

    There are advantages with the bigger pack, you are paying for it, so you know what you're buying.
     
  14. apacheguy

    apacheguy Sig 255, VIN 320

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    Not saying it's the only factor you should consider, but a factor nonetheless. I maintain that kWh absolutely matters.
     
  15. widodh

    widodh Model S R231 EU

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    They could also still tell you how fast it can recharge. The shorter, the better.

    So hence my original question, should there be legislation for this?

    You advertise a number, you also have to sell it.
     
  16. McRat

    McRat Active Member

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    It's a very old argument that started WAY before EV's used anything but lead-acid.

    You buy a car with 400HP. It will not produce 400HP at the tires, nor if you do the math, will it accelerate with 400HP of power.

    You put it on a dyno, and it says 350rwhp. It's not only correct, but perhaps a bit higher than 400.

    Yes, the engine is CAPABLE of applying 400HP or more to an object, but since it's installed in a car, there are drivetrain losses. They didn't lie, the engine IS 400HP.

    People do say once in awhile that MFRs should only report power at the tires. Mazda was caught lying about the output of that RX8? and had to buy back some cars.

    But the current system does work even if sometimes the numbers are wacko. In theory, the Prius has a better Power to Weight ratio (134HP) than the Volt/Ampera (149HP, but much heavier), so it should be as quick if not quicker. Well, the actual sustained power a Prius can put to the ground is far less than 134HP, and it's reflected in the 0-60mph times, and especially the 50-70mph passing times.

    EV range is often tampered with also, which is the number that really affects your EV purchase. What really want out of a car is acceleration, economy, and range. All three numbers are often fantasy from some mfrs.
     

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